With recent breakthroughs in radio communications technologies, there is an increasing demand for electronic parts that can be used at high frequencies ranging from a few hundred MHz to a few GHz or greater. With size reductions of radio communications equipment such as portable telephones, there is also a strong demand for size, and cost reductions of high frequency-conscious electronic parts used with such equipment. To meet these requirements, multilayer ceramic parts are now manufactured by the application of a diversity of integration technologies.
A multilayer electronic part is obtained by co-firing a ceramic material that is an oxide magnetic material and a conductive material, and has one or two or more functions by itself. Such a multilayer electronic part is manufactured by laminating the ceramic and conductive materials one upon another by printing or sheet-making processes to form a laminate, and cutting the laminate according to the desired shape and size followed by firing, or firing the laminate followed by cutting according to the desired shape and size. If required, an external conductor is provided on the electronic part. Thus, this multilayer ceramic part has a structure comprising an internal conductor between ceramic layers. In general, a material such as Ag or Cu is used for an internal conductor suitable for high-frequencies, especially microwaves. With the above production method, however, it has been considered until now that the melting of the internal conductor should be prevented so as to achieve satisfactory properties, and so firing should be carried out at a temperature equal to or lower than the melting point of the internal conductor. Accordingly, it has been believed that a ceramic material fired at elevated temperatures cannot possibly be used in combination with an internal conductor-forming electrical conducting material having a low resistivity yet a low melting point, e.g., Ag, and Cu.
In this regard, the applicant has filed a Japanese patent application (JP-A 6-252618) to come up with a method wherein an internal conductor having a low melting point as mentioned above is formed in a ceramic material unsuitable for low-temperature firing. This is called a conductor melting method wherein an electrical conducting material to form an internal conductor is fired at a temperature that is equal to or higher than the melting point of the electrical conducting material and lower than the boiling point of the electrical conducting material, and solidifying the fired electrical conducting material in the process of cooling. According to this method, the grain boundary between metal grains formed upon the solidification of the molten electrical conducting material becomes as thin as can be regarded as vanishing substantially, and the asperity of the interface between the ceramic material and the internal conductor tends to become small, resulting in a decrease in the high-frequency resistance of the internal conductor and an increase in the Q value at a high-frequency region. Further, a low-cost electrical conducting material having a relatively low melting point, e.g., Ag, and Cu may be used for the internal conductor. Furthermore, it is possible to co-fire the ceramic material and the internal conductor. These are very favorable in view of productivity and cost.